2010A&A...510A..84M

Determining ages of the outer and the inner lobes of so-called double-double radio galaxies (DDRGs) is crucial for understanding the active cycles of galactic nuclei, the phases of interruption of the jet flow, and physical conditions in the surrounding galactic and intergalactic medium governing the jets' propagation. A recognition and understanding of these conditions during the restarted jet activity is of special interest. We determine the ages and other physical characteristics of the outer and the inner lobes of the DDRG J1548-3216, as well as the properties of the surrounding environment during the original and the restarted phase of the jets' activity. Using the new low-frequency and high-frequency radio images of this galaxy, we determined the shape of the spectrum along its lobes and performed the classical spectral-ageing analysis. On the other hand, we applied the analytical model of the jet's dynamics, which allowed us to derive the physical conditions for the source's evolution during the original jet propagation through the unperturbed IGM, as well as those when the restarted new jet propagates inside the outer cocoon formed by the old jet material that passed through the jet terminal shock. The dynamical age estimate of the outer and the inner lobes is 132±28Myr and ∼9±4Myr, respectively. The synchrotron age in the outer lobes systematically rises from ∼25Myr in the vicinity of the lobes' edges to about 65-75Myr in the centre of the old cocoon. These ages imply an average expansion speed along the jets' axis: (0.012±0.003)c in the outer lobes and (0.058±0.025)c in the inner lobes, but the latter speed would be ∼0.25c when they were of age less than 1 Myr. We find that the jet power during the restarted activity is about ten-fold fainter than that of the original jet. Similar disproportion is found for the internal pressures and the magnetic field strengths in the old cocoon and those in the inner lobes. This disproportion can be effectively reduced by assuming the same equations of state for the emitting particles and the magnetic fields within the old and the new lobes. However, we think that our assumption of the non-relativistic equation of state for the old cocoon and the relativistic one for the new lobes is more justified.